1. Field of the Invention
The present invention relates generally to a device to automatically measure the static coefficient of friction (SCOF) on a moving conveyor belt and a means to initiate in-process reinstatement of the SCOF, if a reduction below the design range is detected.
2. Description of the Prior Art
The SCOF is an important parameter in the design of a certain class of machines having conveyors where obtaining necessary accelerations of conveyed parts is completely dependent only on the value of the SCOF, and also where the normal force acting on the conveyor does not exceed the weight of the conveyed part. This conveyor type will be referred to as non positive drive, with no normal force enhancement or for the purposes of discussion, a type three conveyors (A more complete discussion of conveyor types will be explained later in this disclosure)
The reason why this class of conveyor is considered in the design of postal equipment is because this configuration is relatively simple, which yields a design that is low cost and more reliable.
Since the net result of SCOF measurement will directly lead to the cleaning of the conveyor surface, it can be argued that this can be accomplished by regular maintenance procedure on the part of Postal Service personnel. Even though this improvement can be approached in this way (at least theoretically), in practice it can never be optimized when compared to an in-process determination, and also since conveyor cleaning is almost never done in postal facilities since it generally would be done at the expense of mail sorting production time.
This invention is therefore important because it can significantly improve the production efficiency of the type three conveyor, by permitting the use of SCOF values which are significantly greater than those presently being used. The in-process aspect of this procedure will also mean that valuable maintenance time need not be devoted to conveyor cleaning. The potential of a significantly greater SCOF together with the already obvious advantages of low cost and reliability can result in a significant discriminator in improving the competitive position of machines incorporating the invention.
In the design of the type three conveyor production cycle, the acceleration and deceleration portions of the cycle are developed such that selected accelerations do not exceed the value of the SCOF during initial delivery of mail to the conveyor from induction stations, during subsequent constant velocity transporting, or during the critical process of exiting from the carrier cells into the designated sort bins. The net result of not accelerating (or decelerating) the conveyor beyond the limits established by the SCOF, is that synchronismn is maintained throughout the sortation process between the conveyor and the mail being conveyed. In other words, system throughput and sort accuracy are completely dependent on the established minimum acceptable value of the SCOF.
The SCOF is normally measured statically and off-line, by either of several standard (manual) methods. In general, these manual methods are used to obtain data which are then used to define acceptable values for the SCOF, and in turn are then used as a parameter in the dynamics equations of the conveying process. In the design of the SPBS (Small Parcel And Bundle Sorter), the acceleration rates were based upon SCOF values available with new (uncontaminated) belt surfaces which then were reduced by at least 40%, in anticipation of subsequent surface contamination during mail sortation. Conveyor surface contamination is primarily due to a deposition of airborne dust and oil and also embedded debris typical in a postal mail processing facility. This reduction from the ideal value of the SCOF guarantees that synchronous operation will be maintained even in the presence of dirty belts but also results in a significant compromise from the ideal production rates possible if the conveyor belts were consistently maintained at values close to the maximum ideal available value of the SCOF. The single most important advantage of this invention is that it can substantially eliminate the need to reduce the initially higher acceleration rate available, by not permitting the conveyor belt to be significantly contaminated, and thereby realizing the advantage of higher acceleration/deceleration rates which result in proportionately greater throughput and improved sort accuracy.
An objective of this disclosure is to describe a method of SCOF measurement which would be totally automated, and in-process, being performed on a moving conveyor.
A further objective of this disclosure is to describe a method of automated SCOF measurement which is performed under dynamic conditions, as opposed to the traditional manual methods of SCOF measurement which are obtained under static equilibrium conditions. This unique difference in obtaining the value of the SCOF is the key to realizing the benefits of in-process determination of this parameter which directly lead to automated improvement in process control.
Measurement of the SCOF will be remote, using electrical transducers, whose output will be identical to the value obtained by the standard manual methods. The remote measurement of SCOF is not used to the knowledge of the applicants in any postal sorting machine. This in-process, automated scheme would yield the advantage of assessing the SCOF in an ongoing method of evaluating the process in an effort to obtain greater productivity and process control. The information will be utilized to correct the process if a small degradation in the SCOF design range is detected, and also before the process is adversely affected by such degradation. Since the value of the SCOF generally decreases due to contamination of the conveyor drive surface, any method which can dynamically measure the SCOF in-process and which can reinstate the SCOF substantially to its original design range, can significantly improve the productivity of the process. In practice the SCOF measurement obtained with new belts (completely uncontaminated) cannot be completely reinstated after cleaning due to some surface changes resulting from the cleaning process. Actual tests indicate however, that a significant increase in the SCOF is realized as a result of cleaning.
In respect to sorting machines, this benefit will be to improve throughput or sort accuracy. The objective of this disclosure is to describe a method to obtain these benefits.
The SCOF is defined here as the maximum value of the force required to move a piece of mail relative to the conveyor surface, divided by the weight of the mail. Explained in physical terms, it is the result of obtaining the threshold value of slip of the mail relative to the conveyor surface, resulting in the start of undesirable asynchronous movement of mail relative to the conveyor surface. Since this threshold value is a ratio of two forces, the SCOF is a non dimensional parameter.
In general, the acceleration/deceleration portions of the production cycle are designed such that the value of the SCOF is not exceeded since when this threshold is crossed, the dynamic coefficient of friction (DCOF) dominates and its value is significantly smaller than the SCOF. Predictions related to the arrival of mail pieces at defined positions in a sortation cycle, either in successful delivery onto carrier cells from an induction station or in successful exiting of mail into the correct sort bin are best determined under conditions of synchronous delivery. This is due to the fact that this process yields the minimum process time, and therefore, the maximum productivity.
A description of some typical conveyor types used in mail processing is helpful in identifying the specific conveyor category which will benefit the most by this-invention. In general, three types of conveyors are used to transport mail:
(1) Positive drive conveyors using cleats attached to the conveyor surface; the driving cleat provides unlimited acceleration of transported mail, but a constant pitch is required between cleats which has the disadvantage of limiting throughput with varying length packages. PA1 (2) Conveyors which increase the normal force to a value greater than the weight force; this conveyor type also can improve the available acceleration rate substantially as with spring loaded rollers bearing down on the object during transporting, but has the disadvantage of greater complexity and cost. This conveyor type is frequently used with letter mail transporting where letter thicknesses do not vary significantly, but is impractical with packages. PA1 (3) Conveyors which rely strictly on the SCOF for driving objects being conveyed; in this conveyor category, throughput can be maximized for any conveyor speed as compared to the category one conveyor for objects of varying length, since a variable pitch can be developed. This is the simplest conveyor type, however its main limitation is the fact that due the state of the art in conveyor materials, the available acceleration rate cannot exceed approximately 1.0. In practice, a range of only approximately 0.5 to 0.7, is available when the conveyor materials are new uncontaminated and when transporting packages covered with very slippery, and therefore, worst case, packaging materials such as DuPont TYVEK.RTM. brand artificial paper. PA1 (1) a conveyor friction measurement and cleaning system: PA1 (2) a friction measurement unit: PA1 (3) a belt cleaning unit:
It can be shown that the value of the SCOF is exactly equal to coefficient of the acceleration rate available during conveyor operation. A piece of mail being transported on a conveyor at constant velocity is in static equilibrium with the conveyor surface, and is maintained in this synchronous condition by the force of friction: EQU F=(SCOF)N (1)
where SCOF equals the static coefficient of friction, and N equals the normal force between the mail and the conveyor surface, which in the case of conveyor, three is equal to the weight force W.
The force necessary to displace the mail piece relative to the conveyor surface, is governed by Newton's second law: EQU F=(m)(a) or F=(W/g)(a) (2)
Equation (1) can be equated to equation (2), in order to determine the relationship between the SCOF and the acceleration rate, at the threshold between synchronous and non synchronous motion. EQU (SCOF)W=(W/g)(a) (3)
since the weight (W) is on both sides of the equation, it will drop out, yielding the following expression: EQU SCOF(g)=a (4)
This expression indicates that the SCOF determines the value of the acceleration rate (a), in terms of the acceleration of gravity (g), which is a universal constant equal to 386.4 in/sec/sec.
The interpretation of equation (4) is that the value of acceleration (a) that the conveyor and mail piece can be exposed to (without relative motion of the mail with respect to the conveyor surface and therefore maintaining synchronism), is equal to static coefficient of friction (SCOF) multiplied by the acceleration of gravity.
A number of patents can be cited to typify the prior art. For example, U.S. Pat. No. 4,955,933 issued Sep. 11, 1990 to Sistonen discloses a device for measuring the friction on a surface, comprising a measuring wheel and an arm attached to the wheel axle, and a spring attached between the measuring wheel and its axle. The spring is arranged to resist the rotation of the measuring wheel when the measuring wheel is moved by the arm on the surface under measurement. The arm is rigid and is provided with a straight part which is permanently attached to the axle, and at the other end thereof a pull handle is provided. The arm is provided with an inclination indicator. In the measuring position, the straight part of the arm is kept parallel to the surface under measurement. The pull handle is then kept farther away than the radius of the measuring wheel with respect to the surface and is rotatable about an axis parallel to the axle.
U.S. Pat. No. 4,949,574 issued Aug. 21, 1990 to Linden et al. discloses a testing device for vehicle tires and especially anti-skid features situated in the vehicle tire. The tire to be tested is pressed against an outer circle or surface of another tire while the two tires are rotated in opposite directions. The other tire, acting as a counterpart or pair for the tire being tested, is a pneumatic tire, while the tire to be tested and the other tire are disposed to be pressed against one another so that a contact surface between the two tires is substantially straight. The other tire acting as the pair or counterpart for the tire to be tested is provided with a wear surface having good wear resistance properties.
U.S. Pat. No. 4,909,073 issued Mar. 20, 1990 to Takahashi et al. discloses apparatus for measuring a resistance against slippage on the road surface. The apparatus is so constructed that two measuring wheels adapted to be rotated by imparting a tractive force to the apparatus are connected to one another via a torsion bar extending between them. A difference in rotation is forcibly produced by changing the rotational speed of one of the measuring wheels and a slippage resistance on the road surface is measured by detecting a torque generated on the torsion bar due to the slippage resistance on the road surface.
U.S. Pat. No. 4,811,591 issued Mar. 14, 1989 to Antoine discloses a device for checking the surface condition of a material. The device is essentially comprised of an adherence measuring sensor whose sensitive element is set in contact with the surface of the material while in relative motion with respect to the surface. An associated computer provides for the comparative measurement, the display and the possible piloting of the surface treatment or production apparatus. The sensor comprises two wheels mounted on fluid bearings and rolling on the material of which the surface condition is to be controlled and devices for measuring the speed differential of the wheels and providing for the progressive braking of one of the wheels.
U.S. Pat. No. 4,662,211 issued May 5, 1987 to Strong discloses a wheeled vehicle constructed to be propelled along a test surface in a predetermined direction. A test wheel assembly includes a test wheel carried by a hub to rotate about an axis perpendicular to the direction of vehicle travel, with transducers being mounted on the hub and coupled to the test wheel for dynamically measuring forces acting on the test wheel horizontally in the direction of travel and vertically perpendicular to the direction of travel. A drive mechanism is coupled to at least one wheel of the vehicle and to the test wheel for driving the test wheel at predetermined slip with respect to the vehicle wheel. An important feature resides in the construction according to which the drive mechanism is suspended from the vehicle frame and held against rotation about the axis of the vehicle wheel coupled thereto, and the test wheel transducer is suspended from the drive mechanism and held against rotation with respect to the test wheel axis. In this way, transducer orientation is maintained independently of undulations in the test surface.
U.S. Pat. No. 4,594,878 issued Jun. 17, 1986 to Abe et al. discloses a dynamic friction coefficient measuring apparatus. It includes a friction measuring portion having a disc with a friction measuring rubber member attached thereto, a driving disc adapted to rotate coaxially with the disc and a dynamometer which interconnects the disc and the driving disc. A tachometer measures the speed of the rubber member during rotation of the friction measuring portion. An X-Y recorder records two electric outputs of the friction measuring portion and the tachometer onto rectangular coordinates.
As disclosed in U.S. Pat. No. 3,367,170 issued Feb. 6, 1968 to Lynch et al., to measure the coefficient of friction of a surface, a body having a shiftable center of gravity is pulled or pushed along the surface. A scale is attached to the body. When the center of gravity has shifted enough to create a restoring force equal to the moment introduced by the friction force, the reading on the scale at that instant is the coefficient of friction of the surface.
U.S. Pat. No. 3,152,468 issued Oct. 13, 1964 to Powell discloses a tire testing system wherein a drive shaft rotates a drive gear and a tire-driving drum. A tire in engagement with the drum is connected by a shaft to a side gear of a differential. The other side gear of the differential is connected to a spur gear connected through an idler gear to the drive gear. The rear ratio of the drive gear to the spur gear is equal to the diameter ratio of the drum and the tire so that the ring gear of the differential normally remains stationary, regardless of the speed of the tire. By urging the ring gear in one direction the tire is subjected to braking forces. Rotation of the ring gear in the other direction subjects the tire to acceleration forces.
U.S. Pat. No. 2,990,713 issued Jul. 4, 1961 to Heffelfinger et al. discloses apparatus which relates to the measurement of the frictional properties of various materials, and is especially useful in determining the inter-fiber dynamic and static frictional properties and stick-slip characteristics of textile fibers. Fundamentally, the invention operates on the principle of a brake band, and in practice the sample under test is composed of two parts which are pressed together with a known pressure and moved relative to each other. In effect, the two sample parts represent a brake drum and belt, and the resistance to the movement of the parts relative to each other is measured and recorded.
It was with knowledge of the foregoing state of the technology that the present, invention has been conceived and is now reduced to practice. The concept embodied by this invention is different from all of the devices reviewed above. Specifically, none of the patents found in the search disclose measuring the current of the motor, especially the maximum current of the motor, in order to determine coefficient of friction. Furthermore, the patents relating to aircraft wheels are not applicable to conveyor belts because of differences in the surfaces. An aircraft tire is subjected to relatively hard impacts with the runway, resulting in removal of some of the tread of the tire and thus renewing the contacting or working surface of the tire. In contrast, mail pieces land on a postal conveyor relatively and very desirably so as to avoid damage, softly. For this reason, the conveyor surface would not be renewed as would that of an aircraft tire. The scope of the invention is broad enough to consider using an intermediate layer of the "worst-case" mail piece external material, TYVEK.RTM. brand artificial paper, to make the coefficient of friction measurements. Also, within the scope of the invention is making the roller of highly polished or hardened steel. A soft roller would tend to receive foreign substances more readily than would a roller of hard material and thus would produce variations in its surface and would vary the coefficient of friction being measured.